The present invention relates to a method of quickly predicting the degree of nodularity of spheroidal graphite cast iron from a molten iron sample and an apparatus thereof, particularly relates to a method and an apparatus thereof, wherein the relation between temperature and time of a sample of molten iron for spheroidal graphite cast iron during cooling and solidification is measured by thermal analysis, three parameters strongly correlated to the degree of nodularity of spheroidal graphite cast iron are substituted into a formula previously statistically obtained from a series of experiments, and a result is obtained by operational process.
Spheroidal graphite cast iron has many excellent characteristics such as mechanical and physical properties, weldability, machinability and the like as compared with flaky graphite cast iron, but many problems as to manufacture remain unsettled.
That is, the degree of nodularity of molten iron for spheriodal graphite cast iron and further the quality of a product may be determined by the property of base iron, which is influenced by, for example, kind of melting raw material, chemical composition, history of melting treatment, melting process or the techniques of nodulizing treatment which is influenced by for example treated amount, treating temperature, treating procedure, kind and amount of added nodulant, etc. Thus, as compared with the flaky graphite cast iron, the spheroidal graphite cast iron has many manufacturing factors influencing upon the quality of a product, so that it is very difficult to positively control all these manufacturing factors.
In general, the degree of nodularity of spheroidal graphite cast iron is predicted directly by the percentage of spheroidal graphite based on observation of the structure of a sample cast under the same condition as castings or a sample piece attached to the casting itself through a microscope or determined indirectly from the mechanical properties such as tensile strength, elongation or the like, percentage of residual magnesium added as nodulant (residual Mg amount) and the like, which are significantly correlated to the degree of nodularity.
The determination based on the above measurement results is reliable but both the measurement and the determination take a long time, and even the observation of structure, which is deemed to be most quickly made, takes several hours after pouring, and when its degree of nodularity is predicted as bad, pouring would be ended and it is too late to take necessary action, a casting turns out inferior, and an economical loss is fatal in manufacture.
Casting industry has, therefore earnestly asked for the development of a method for quickly and precisely predicting the degree of nodularity of molten iron for spheroidal graphite cast iron (hereinafter referred to as SG molten iron), i.e., the nodularity after solidification of the molten iron before pouring the iron into molds immediately after nodulizing treatment to take necessary measures without delay.
As well known, the degree of nodularity of molten iron at the time of casting depends basically upon a residual Mg amount in molten iron or percentage of spheroidal graphite in the solidified state immediately after a nodulizing treatment. The decrease in a residual Mg amount or the lowering of the percentage of spheroidal graphite from the nodulizing treatment to the pouring depends upon the standard working conditions of each foundry shop, such as a maintaining temperature and time of the molten iron, a shape and a size of a ladle used and the like. Therefore, if the degree of nodularity of molten iron for spheroidal graphite cast iron immediately after a nodulizing treatment can be predicted, whether to be poured or not to be poured can be determined on the basis of the standard working conditions of each foundry shop and it becomes possible to avoid any reject manufactured by pouring an ill-treated molten iron.
In the process of cooling and solidifying the SG molten iron, a shape of a cooling curve obtained by thermal analysis (a curve showing the relation between the lapse of time and the temperature of a sample) closely relates to the degree of nodularity of spheroidal graphite cast iron after solidification, so that the so-called thermal analysis method is proposed recently; namely, an SG molten iron sample immediately after nodulizing treatment is tested by thermal analysis and the degree of nodularity of the SG molten iron is predicted from the differences of the process of temperature change before pouring into a mold. For example, a molten iron sample taken from the SG molten iron is poured into a sample mold (cup), a cooling curve is record-traced with the use of a suitable thermoelectric pyrometer and the degree of nodularity of the SG molten iron is predicted from the differences in shape of the curve obtained. This method is, however, to compare the curve with more than several tens of classification beforehand prepared with respect to the shapes of cooling curves and the degrees of nodularity of many examples, and to satisfy quickness of measurement, but comparison and analysis are complicated and troublesome, so that there is the possibility of being occupied by a subjective point of view and making a large error, and as a result, this method is not practically used at foundry shops.
As another method, a cooling curve being utilized in the same manner, a primary crystallization temperature, the lowest temperature by undercooling and the highest temperature due to recalescence subsequent to the undercooling, both of which occur at the time of eutectic solidification, are measured from the curve with the eye, and from a relation with the temperatures thus obtained by eye measurement the degree of nodularity of SG molten iron and carbide content are predicted. According to this method, quickness of measurement can be satisfied in the same manner as in the former method, but inaccurate readings of the lowest temperature and the highest temperature caused by a pyrometer which does not display numerical values result in an error, and, furthermore, resolving power of thermoelectromotive force of the pyrometer is insufficient and time parameters of temperature change, which are important for predicting the degree of nodularity of SG molten iron, are not taken into consideration, so that this method cannot obtain sufficient precision and is not practically used at the foundry shop.
Further, as a further method of utilizing a cooling curve, the degree of nodularity of SG molten iron is estimated from a single correlation with such as the lowest temperature by undercooling, the highest temperature due to recalescence, which occur at the time of eutectic solidification, a difference between both the temperatures, the maximum inclination angle of a curve from the lowest temperature to the highest temperature or the like. According to this method, as well as the two preceding methods, quickness can be satisfied, but there are an error for reading the lowest and highest temperatures, insufficient resolving power of thermoelectromotive force and deficient prediction standard with the use of a single correlation only, so that sufficient precision cannot be obtained, and as this method can only be applied to hypoeutectic SG molten iron, while almost all foundry shops manufacture spheroidal graphite cast iron from hyper-eutectic molten iron at present, this method is impossible to be used at the foundry shop.
At all events, as to prediction of the degree of nodularity of SG molten iron, if reliability has priority, it takes a long time for obtaining a result. In a conventional method with the aid of a thermal analysis method, quickness of measurement can be satisfied, but sufficient reliability cannot be obtained for the reason mentioned in the preceding. Further, the applicable composition of molten iron is limited, so that there is no method practically usable at the foundry shop.